Since a nonaqueous electrolyte battery such as a lithium primary battery and a lithium ion secondary battery is light and capable of obtaining a high voltage, and has a high energy density, it has been used in a wide range of fields. Here, an example of a nonaqueous electrolyte secondary battery among nonaqueous electrolyte batteries of related art is described referring to FIG. 4.
FIG. 4 is a perspective view showing a cylindrical nonaqueous electrolyte secondary battery produced conventionally by sectioning the battery longitudinally.
The nonaqueous electrolyte secondary battery 10 uses a wound electrode body 14 produced by winding a positive electrode 11 and a negative electrode 12 through a separator 13, and has such a constitution that: insulating plates 15 and 16 are disposed respectively on the top side and bottom side of the wound electrode body 14 to prepare a parts set; the parts set is held in the inside of a steel-made cylindrical battery outer can 17 serving also as a negative electrode terminal; a power collecting tab 12a of the negative electrode 12 is welded to an inside bottom of the battery outer can 17, and a power collecting tab 11a of the positive electrode 11 is welded to a bottom plate of a current-intercepting opening-sealing body 18 with a built-in safety device; a predetermined nonaqueous electrolyte is poured through an opening of the battery outer can 17; and the battery outer can 17 is sealed with the current-intercepting opening-sealing body 18 after pouring the nonaqueous electrolyte. Such a nonaqueous electrolyte secondary battery exhibits the excellent effects of, for example, high battery performance and battery reliability.
As a negative electrode active material used in the nonaqueous electrolyte secondary battery, carbonaceous materials such as graphite and amorphous carbon are widely used. On the other hand, as a positive electrode active material, a lithium-transition metal compound oxide represented by Formula: LixMO2 (in which M represents at least one of Co, Ni and Mn) capable of reversibly intercalating and deintercalating lithium ion is used. Examples of the lithium-transition metal compound oxide include LiCoO2, LiNiO2, LiNiyCo1-yO2 (y=0.01 to 0.99), LiMnO2, LiMn2O4, LiCoxMnyNizO2 (x+y+z=1) or LiFePO4, and these compound oxides are used individually or in combination.
Further, as a solvent (organic solvent) of a nonaqueous electrolyte, carbonates, lactones, ethers and esters are used individually or in combination of two or more thereof. Among them, particularly carbonates having a large dielectric constant and having a large ion conductivity of the nonaqueous electrolyte thereof are frequently used. Here, as a solute of the nonaqueous electrolyte, LiPF6, LiBF4, LiCF3SO3, LiN(CF3SO2)2, LiN(C2F5SO2)2, LiN(CF3SO2(C4F9SO2), LiC(CF3SO2)3, LiC(C2F5SO2)3, LiAsF6, LiClO4, Li2B10C10, Li2B12Cl12 or a mixture thereof is used.
Thus, in the nonaqueous electrolyte battery, a flammable organic solvent is used as a solvent of the electrolyte. Therefore, for avoiding a danger in a case where the temperature inside the battery is excessively elevated due to an internal short circuit etc. by an external short circuit, overcharge, improper connection, battery drop etc., normally in the nonaqueous electrolyte battery, a safety apparatus such as a safety valve, a PTC (Positive Temperature Coefficient) element, and a current controlling circuit is provided.
Further, a separator used in the nonaqueous electrolyte battery assumes the important function of “maintenance of electronic insulating properties” for preventing the internal short circuit of the battery. It is known that the separator has a large influence on the battery characteristics and safety. Namely, the separator must be able not only to prevent the short circuit in the positive and negative electrodes in an usual using condition of the nonaqueous electrolyte battery, but also to maintain the battery voltage even in a high-load condition thereof by suppressing the electric resistance to low by a porous structure thereof. In addition, the separator must have a shutdown function for suppressing an excessive temperature elevation of the battery by causing the separator to be substantially imperforated while maintaining the predetermined size of the length and width of the separator to enlarge the electric resistance and to terminate the battery reaction in the case where a large current has been passed through the nonaqueous electrolyte battery due to an internal short circuit etc. by an external short circuit, improper connection or battery drop etc., and the battery temperature has been elevated. Therefore, as a separator for the nonaqueous electrolyte battery according to related art, a fine porous film composed mainly of a polyethylene resin or a fine porous film composed mainly of a polypropylene resin has been frequently used (see JP-A-8-244152 and JP-A-2002-279956).
Thus, though the separator for the nonaqueous electrolyte battery has assumed an important function up until the present, there has been no unitary standard among the manufacturers for quantifying the property “maintenance of electronic insulating properties” necessary for the separator for nonaqueous electrolyte batteries and the evaluation of the separator for nonaqueous electrolyte batteries by measuring the appropriate physical properties is performed for every manufacturers. For example, in JP-A-11-102683, there is disclosed an invention of a method for evaluating a porous film by heating a porous film of a separator etc. using a heat sealer at 50 to 200° C. under a pressure of 0.1 to 50 kg/cm2 for 0.1 to 60 seconds and by measuring an alternating current resistance of the film in an organic electrolyte using an ohm meter to calculate the electric resistance.
The above-described evaluation method of a porous film of related art includes: pressurizing and heating the porous film of a separator etc. at a certain temperature, under a certain pressure and for a certain time; cutting out a predetermined part from this pretreated porous film; fixing the predetermined part in an electric resistance measuring cell containing an organic electrolyte to measure the alternating current resistance thereof. This measuring method is performed for the purpose of measuring a thermal behavior of the separator and is not suitable for evaluating the quality and reliability with respect to the “maintenance of electronic insulating properties” of the separator under the circumstances in which the constituting pressure within the battery is varied due to the expansion and contraction of the electrode plate according to the charge and discharge. Because for measuring the relationship between the time for pressurizing or heating at various temperatures or under various pressures and the alternating current resistance, it is necessary to prepare porous films which have been subjected to several types of pretreatments in which the pressurizing time or the heating time is variously changed and to measure the alternating current resistances of these pretreated porous films respectively, there is the problem that the measurement takes much time. In addition, for measuring the alternating current resistance when both the heating temperature and the pressurizing pressure of the porous film are varied, even more pretreated porous film samples are required to be prepared and measured. Hence, it is apparent that the measurement inevitably takes a lot longer.